What is the range of the device antenna (inside antenna)?

The range or distance that your phone can extend from a device antenna (sometimes called the inside antenna) without affecting your connection depends on two specific factors: (1) the strength of the signal from the cell tower, and (2) the output power of your signal booster.

The stronger the tower signal is BEFORE it's amplified by the signal booster, the greater range you will have from the device antenna.

Also, a more powerful signal booster, which can provide a higher level of signal amplification, allows greater range. Hopefully from this answer you understand that range has to do with more than just the antenna itself.